A spark plug is used for ignition of an internal combustion engine such as an automobile engine. The spark plug generally includes a tubular metallic shell, a tubular insulator disposed in an inner hole of the metallic shell, a center electrode disposed in a front side inner hole of the insulator, and a ground electrode joined at one end thereof to the front side of the metallic shell with a spark discharge gap provided between another end of the ground electrode and the center electrode. The spark plug causes spark discharge at the spark discharge gap formed between the front end of the center electrode and the front end of the ground electrode in a combustion chamber of an internal combustion engine, to burn fuel with which the combustion chamber is filled.
Meanwhile, a Ni alloy or the like is generally used as a material forming a ground electrode and a center electrode. A Ni alloy is slightly inferior in oxidation resistance and wear resistance to a precious metal alloy containing a precious metal such as Pt or Ir as a main component, but is suitably used as a material forming a ground electrode and a center electrode since Ni is cheaper than a precious metal. However, in recent years, the temperature in a combustion chamber tends to increase. When spark discharge is caused between the front end of a ground electrode and the front end of a center electrode which are formed of a Ni alloy or the like, the respective opposed front ends of the ground electrode and the center electrode are likely to cause spark wear. Thus, a method has been developed in which a tip is provided at each of the opposed front ends of the ground electrode and the center electrode such that spark discharge is caused at the tip, thereby improving the wear resistance of the ground electrode and the center electrode. As a material forming the tip, a material containing, as a main component, a precious metal that is excellent in oxidation resistance and spark wear resistance is often used.
For example, Patent Document 1 states that an object of “the present invention is to provide a higher-durability spark plug . . . , in which spark wear, oxidation wear, and abnormal wear of the precious metal member are suppressed, and a phenomenon of occurrence of spherical projections on the precious metal member is suppressed” (see lines 11 to 15, page 4 of Patent Document 1), and describes “a spark plug . . . , the precious metal member contains Ir as a main component, and contains not less than 0.3 mass % and not greater than 43 mass % of Rh, not less than 5.2 mass % and not greater than 41 mass % of Ru, and not less than 0.4 mass % and not greater than 19 mass % of Ni”, as means for achieving the object (see claim 1 of Patent Document 1).
In addition, Patent Document 1 states that “In order to sustain superiority in another condition of use, for example, to further improve oxidation wear resistance at a high temperature (900° C. or higher), for example, Pt, Pd, Re, or Os can be contained in the precious metal member. Alternatively, in order to sustain superiority in another condition of use, for example, to further improve oxidation wear resistance and spark wear resistance in the case where the temperature of the plug (precious metal member) is relatively low (about 600° C.), an oxide (including a composite oxide) of an element selected from Sr, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Ti, Zr, and Hf can be contained in the precious metal member. Particularly, Y2O3, La2O3, ThO2, or ZrO2 is preferably used” (see lines 39 to 47, page 4 of Patent Document 1).